to make it look smoooth
you still have all three parts welded together, will probably make it stronger
denee: i agree with kim, it looks real thin on top from rust.
think seriously about shaving your cowl vent too, that's where the water and rust come from.
you can also weld the inner cowl and the firewall from the inside, it adds a little meat to the metal.
this pic you can see where the welding from the inside burnt the paint
other than thin metal, your doing it right. just keep welding until you find metal.
if it keeps burning back maybe a filler piece will be needed.